If you join your ranked team before September 1st, 2012 and your team already have gold, then you automatically get Gold without playing 5 ranked games.

If you join your ranked team after September 1st, then you need to WIN 5 ranked games to get your ranked team's reward (bronze,silver,gold,platinum, and diamond).

If your team have gold on 3v3 elo rating and bronze on 5v5 team and you play and win 5 ranked games on 5v5 then you still don't get gold from 3v3. You have to play and win 5 ranked games on 3v3 to get your team elo reward.